Known actuating drives can be operated with, for example, hydraulic, pneumatic or electrical energy. A control fitting can be a valve or flap which is inserted into a pipeline and more or less allows or blocks the flow of a process medium through this pipeline as a function of a predefined setpoint value. To this end, the actual position of the control fitting can be determined with the aid of the travel measurement system and compared with a predefined setpoint value. The setpoint value can be provided by a superordinate device and electrically transmitted to the position regulator.
Operating instructions for an electropneumatic position regulator TZIDC from ABB, document 42/18-84-DE, issue 01.2009, disclose a known position regulator having a regulation unit, a travel measurement system and an electropneumatic converter having a signal shaper which is called an expansion module and which is connected to the regulation unit and is suitable for outputting the determined position via a separate communication connection. The signal shaper is separately fed via a conductor loop which also serves for communication with a superordinate device. The regulation unit provides the signal shaper with a travel signal which corresponds to the actual position of the control fitting. This travel signal is converted, in the signal shaper, into a conduction signal which can be transmitted to the conductor loop as a shaped (e.g. impressed, or superposed) current.
The travel signal is no longer available in the event of a disturbance in the regulation unit. Although an alarm level is received by the regulation unit and output to the conductor loop, the actual position of the control fitting is not available in the superordinate device.
DE 10 2006 049 651 A1 discloses redundantly supplying power to a position regulator from two electrical feeds in order to maintain the availability of the position regulator. Although this compensates for simple failures in the power supply and maintains operation of the position regulator as such, the transmission of setpoint values is also interrupted in position regulators which are connected to a two-wire line which serves both for feeding and communication for example, for transmitting the setpoint value via an impressed current, when feeding via this two-wire line fails.
The disclosures of the foregoing documents are incorporated herein by reference in their entireties.